The Gem. says that the Bnos Yisrael were machmir that if they say ''tipas dam k'cherdal'' they would keep Shiva N'kiyim.
The question is, if the Din of the Torah didn't require it, and the Chochomim also didn't, what does it mean to say that the Bnos Yisrael were machmir and took these stringencies on. How did they have the authority to have this practice become standard observance? Normally, a person is bidden not to be strict if there is no halachically-sound reason for doing so. The Chochomim obviously didn't see the need to impose this; it came from the woman themselves.
Also, how does this fit with the concepts of ''Ba'al Tosef'' and also ''Koach D'Heteira Adif''?
The Bnos Yisrael did not have the authority to change the Halachah. However they instituted this practice as a safeguard against those who would mistake Dam Zivah for Dam Nidah. This Chumrah falls into the category of Minhagei Isur: although one cannot change the Halacha, a Kehilah may institute a Minhag if they feel that it will strengthen the adherence to a Mitzvah.
Into this category fall Minhagim like not doing Melachah on Erev Pesach or Kitniyos on Pesach etc. With certain restrictions, a Kehillah may institute such a Minhag and it is binding on all future generations. This is not to be confused with a stringency in Halachah; here those instituting the Minhag know that it is not halachah. Rather it is a Minhag which the times require.
In general there is no Ba'al Tosif when one's intent is not to add a Mitzvah or a portion of a Mitzvah. When it is clear that what he is doing is merely a Minhag there is no Ba'al Tosif. Also, generally Ba'al Tosif does not apply to Mitzvos Lo Ta'aseh .
As for Ko'ach d'Heteira Adif , this is one of the most misunderstood (or deliberately twisted) ideas in all of Chazal. Chazal do not favor Kulos over Chumros. Each question must be weighed properly in order to deduce the truth. Chas v'Shalom to say that Chazal were looking to be lenient, or stringent.
Rather, the idea of Ko'ach d'Heteira Adif, is that it is more difficult to be Mekil. As Rashi explains in Beitzah, if one wants to "play it safe" he has to be Machmir, but to be Mekil he has to be sure. Therefore the Mishnah oftentimes, when having a choice to show either the extent to which the Machmir is Machmir or the extent to which the Mekil is Mekil, will choose the latter, since it is more difficult to be Mekil that is the greater Chidush.
Dov Zupnik
You noted that the statement from Beitza of Koach d'heteira adif, is very misunderstood. Indeed it is! And while time doesn't permit a lengthy note (I am about to give shiur) suffice it to say you should look at:
- Rashi Kesuvos 7a
- Teshuvos Maharashdam Y.D. end of 192
- Mishnah Yadayim 4:3 with Tiferes Yisrael
- Kovetz He'aros 67:1
The Chochomim obviously didn't see the need to impose this; it came from the women themselves. However the Chachomim did accept it and made it Halacha.
Regarding the Kollel's remark-
>>Also, generally Ba'al Tosif does not apply to Mitzvos Lo Ta'aseh.<<
This is a Machlokes between the Rambam and the Raavad Hil. Mamrim 2:9, (and see at length in Mamar Loi Sosif in the Divrei Nvi'im of the MaHaRaTZ Chayos).
Kol Tuv
Yitzchok Zirkind